1. Field of the Invention
The present invention relates to coarse frequency estimation of signals, having been transmitted by an IEEE 802.11a based Orthogonal Frequency Division Multiplexing (OFDM) transmitter, by an OFDM receiver for correction of frequency errors in the received OFDM signals.
2. Background Art
Local area networks historically have used a network cable or other media to link stations on a network. Newer wireless technologies are being developed to utilize OFDM modulation techniques for wireless local area networking applications, including wireless LANs (i.e., wireless infrastructures having fixed access points), mobile ad hoc networks, etc. In particular, the IEEE Standard 802.11a, entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band”, specifies an OFDM PRY for a wireless LAN with data payload communication capabilities of up to 54 Mbps. The IEEE 802.11a Standard specifies a PHY system that uses fifty-two (52) subcarrier frequencies that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM), or 64-QAM.
Hence, the IEEE Standard 802.11a specifies an OFDM PHY that provides high speed wireless data transmission with multiple techniques for minimizing data errors.
A particular concern in implementing an IEEE 802.11a based OFDM PHY in hardware involves providing a cost-effective, compact device that can be implemented in smaller wireless devices. Hence, implementation concerns typically involve cost, device size, and device complexity.
One particular concern involves frequency differences (fE) between the transmit frequency (fT) generated by a local crystal oscillator in the OFDM transmitter and the receive frequency (fR) generated by the local crystal oscillator in the OFDM receiver. The resulting frequency error (fE=fT−fR) may cause substantial deterioration of the signal to noise ratio if left uncorrected.
The IEEE Standard 802.11a specifies a short preamble and a long preamble that may be used by the OFDM receiver for generating an estimated frequency error (fEST). In actual implementation, however, the estimated frequency error (fEST) does not equal the actual frequency error (fE) because both the short preamble and long preamble contain noise components from transmission between the OFDM transmitter and the OFDM receiver. Hence, the short preamble and long preamble received by the OFDM receiver differs from the short preamble and long preamble output by the OFDM transmitter.
FIG. 1 is a diagram of a preamble 10 used by an OFDM receiver for synchronization with an 802.11a OFDM packet 12, reproduced from FIG. 110 (Section 17.3.3) of the IEEE Standard 802.11a. In particular, the preamble 10 is a Physical Layer Convergence Procedure (PLCP) preamble having a first training portion (i.e., a short preamble) 14 and a second training portion (i.e., a long preamble) 16. The first training portion 14, typically used for signal detection, automatic gain control, diversity selection, coarse frequency offset estimation, and timing synchronization, includes ten (10) identical short preamble symbols (t1, t2, . . . t10) 18; each short preamble symbol 18 is implemented as a 16-sample symbol. The second training portion 16 includes long training symbols (T1 and T2) 20 and a guard interval (GI2) 22. The second training portion 16 typically is used for channel and fine frequency offset estimation.
FIG. 2 is a diagram illustrating a typical approach in performing a coarse frequency estimation in a conventional 802.11a system. In particular, after automatic gain control (AGC) has been performed by the OFDM receiver at event 28, coarse frequency estimation is performed by performing autocorrelation on the short preamble samples 32 using a fixed-size (e.g., 32 sample) correlation window 30 over the short preamble samples 32, where each short preamble symbol 18 includes sixteen (16) samples 32. As illustrated in FIG. 2, the first usable sample following AGC completion at event 28 is the sample x0: the fixed-size window 30 shifts in the direction 34 as new samples (e.g., xn) 32 are received by the OFDM receiver. Hence, the autocorrelation result A(n) is performed according to:
                              A          ⁡                      (            n            )                          =                              ∑                          k              =              0                        15                    ⁢                                                    x                                  n                  -                  k                                            ⁡                              (                                  x                                      n                    -                    k                    -                    16                                                  )                                      *                                              (        1        )            
Where the symbol “*” represents a complex conjugate (i.e., x* is the complex conjugate of x).
The above-described approach of using a fixed-size window 30 for autocorrelation to determine coarse frequency estimation suffers from the disadvantage that the short preamble samples 32 may have different values due to noise, adversely affecting the accuracy of the coarse frequency estimation in noisy conditions.